Electromagnetic flowmeter: principle of operation and metrological characteristics

Electromagnetic flowmeter is a technological device that is widely used to account for the flow of coolant, measurement of liquid and gaseous substances. Undoubted advantages of such systems are: the implementation of the design with the use of a minimum number of mechanical elements, the lack of hydrodynamic resistance, the highest accuracy of recording indicators.

Electromagnetic flowmeter - principle of operation

Devices of this category are equipped with conductors, in which, thanks to the intersection of the magnetic field with the lines of force, an electromotive force is produced. The current direction in the conductor is perpendicular to the direction of the magnetic field. This regularity is fully disclosed in the so-called Faraday law, which explains the principle of electromagnetic induction.

When the conductor is replaced by a flow of liquid conducting electricity, a schematic diagram is obtained according to which the electromagnetic flowmeter actually acts. Such devices can be equipped with permanent or electric magnets that are powered by alternating current.

In the measurement area of the flowmeter, a pipe made of non-magnetic non-conductive material is placed. Often, this zone is equipped with insulating inserts of inert plastic. The use of such elements helps to obtain the most accurate indications in the process of measuring the characteristics of the working medium.

Fields of operation

In which areas do you use an electromagnetic flowmeter? The most widespread such devices are in the field of energy and water accounting. In particular, such devices are used in the arrangement of heating systems.

Currently, the electromagnetic flowmeter is widely used in the biochemical, metallurgical, food industry. The use of devices of this category resorts to construction organizations, ore-dressing enterprises, medical institutions.

Electromagnetic flowmeter is indispensable in the organization of automatic control systems for flow of working media, where the main role is played by the degree of delay in measuring indicators. Applied the device when tracking the rapidly changing costs of liquid and gaseous substances.

Metrological errors

As the long-term practice of operating measuring instruments with a constant magnetic field shows, their error averages on the order of 1-2.5%. The specified minus becomes appreciable at measurement of indicators of weakly pulsating environments. In this case, there are often additional errors caused by the polarization of the electrodes, which is reflected in the change in the resistance of the device converter.

Reducing the percentage of metrological errors in flowmeters with a constant magnetic field is facilitated by the realization in the design of calomel and carbon electrodes or conductors with platinum and tantalum coating.

As for devices with an alternating magnetic field, there is no polarization of the electrodes, which can lead to errors in the indices. However, such devices have other effects that distort the signal:

1. Transformer effect - when liquid passes through conductors that form turns, connecting wiring and electrodes form a transformer electromotive force. To compensate for this action, magnets with a constant switching current or compensating circuits are introduced into the measuring circuit of flowmeters with an alternating magnetic field.
2. Capacitive effect - arises because of the significant difference in potential between the connecting wiring and the system responsible for the formation of the magnetic field. Errors in this case are compensated for by careful screening of the elements of the system.

Checking electromagnetic flowmeters

How is the verification of electromagnetic flowmeters? The most accurate method here is static weighing (mass method). For its implementation, use an exemplary balance and shut-off valve, which operates in the "start-stop" mode.

In the course of the work, the net weight of the working fluid is determined according to the measurement of the weight of the container before and after its filling as the medium passes through the flowmeter during a certain time interval.

Advantages of the verification method are:

• Accuracy of measurements of normalized flow indicators;
• The possibility of correlation of results with values corresponding to state standards;
• Possibility of verification of the flowmeter, which works with hot liquids;
• Process automation.

Types of flowmeters

In addition to electromagnetic flowmeters, there is a wide variety of other instruments for measuring the characteristics of working media passing through pipelines. Currently, based on the principle of operation, the following groups of devices are distinguished:

• Thermal;
• Ultrasound;
• Vortical;
• Coriolis;
• Mechanical;
• Micro-flowmeters.

In the following, we briefly review the features of electromagnetic flowmeters of each type.

Thermal flowmeters

The principle of operation is based on a local change in the properties of the working fluid, for example, the temperature in the flow with the subsequent measurement of the indicators at a remote site from the site of impact. This method contributes to the calculation of the average velocity of the substance along the conducting paths.

Similarly, other characteristics of the substance, for example, the chemical composition, can be changed. However, in most cases this approach is unacceptable, most often when operating flow meters in medical facilities.

Ultrasonic flowmeters

The functioning of instruments of this type is based on the ability of sound waves to propagate in a mobile medium. By determining the source of ultrasound and its receiver, it is possible to judge the velocity of the flow of the working medium in accordance with the indices of the movement of the wave over a certain interval.

Vortex flowmeters

In the devices of this plan, the main functional element is a spherical or disk-shaped target. The part is fixed on a fixed elastic cable. When passing through the system, the flow of the working medium affects the target, which leads to its displacement. This, in turn, causes deformation of the cable, the changes on which are fixed by special strain gauges. The information obtained facilitates the formation of judgments regarding the direction and velocity of the flow of matter.

Coriolis flowmeters

Structurally, such devices consist of a tube that is exposed to vibrations that emanate from an external generator. In the absence of fluid in the system, oscillations cause simultaneous acceleration of all sections of the tube. As the liquid passes through it, the so-called Coriolis force enters into action , which is directed in opposite directions for the input and output flow of matter. This leads to a shift in the phase of vibration of the conductive tube and makes it possible to record the necessary parameters.

Micro-flowmeters

To this category of measuring devices are flowmeters, which differ in their miniature performance. The dimensions of this or that device are determined by the scope of its application. Such devices meet the needs of medical institutions and enterprises for the production of chemical products.

According to the principle of operation, each miniature device is a conventional flow meter - an electromagnetic counter. However, due to the possibility of exploitation in cramped conditions, the cost of such devices is an order of magnitude higher.

Mechanical flowmeters

This group includes devices that are structurally devoid of electronic components. The flow velocity here is measured by the action of the medium on the mechanical turbines.

Despite the affordable cost of such flowmeters, their accuracy leaves much to be desired. Another disadvantage is the use of moving parts, which can become an obstacle to the movement of gaseous or liquid substances. However, despite the indicated minus, mechanical flowmeters find the widest application in domestic conditions when it is necessary to account for the flow of water.

Electromagnetic Flowmeters - Common Models

Let's look at the demanded electromagnetic flowmeters, which are in greatest demand on the domestic market.

The electromagnetic flowmeter "PREM" is designed to measure the volume and flow of electrically conductive liquid substances. The readings are displayed on external, user-friendly devices. Such devices are suitable for operation in the conditions of large industrial complexes, they are used for servicing objects of housing and communal services (in the system of accounting for water consumption and thermal energy).

The electromagnetic flowmeter Promag is suitable for recording indicators of working media with a high temperature. It is a complex functional electronic device. It is used mainly in the composition of modular structures in the implementation of high-tech processes in industrial areas.

"Peterflow" - an electromagnetic flowmeter designed to account for the volume and volume flow of fluids transported through a pipeline. The devices of this brand are in demand in the sphere of heat power engineering, housing and communal services, and in industry. Distinctive features of such measuring devices are: the placement of electronic equipment in a separate, well-protected case, the availability of protection against unauthorized access, the implementation of convenient graphic displays with backlighting.

Electromagnetic flow meter "ERSV VZLET" is currently the most widely used metering device at housing and utilities facilities. Applicable for recording the volumetric flow of both cold and hot water.

The electromagnetic flow meter "VZLET" is distinguished by the absence of pressure loss on the measured sections of the pipeline, it does not need the installation of additional filters. Information about the indicators can be output to the frequency, pulse or logical output, depending on the needs of the user. The flow meter "RISE ER" electromagnetic has a function of controlling the emptying of the pipeline. The only comparative disadvantage of such systems is the need for mounting on rectilinear sections.

Finally

Apparently, the electromagnetic flowmeter is an extremely accurate, functional measuring device. Primary converters of such devices do not contain elements protruding into the pipeline, texture parts and profile constrictions. These features provide minimal errors in the indications. Among other things, electromagnetic devices allow for the cleaning and maintenance of pipelines without dismantling the elements of the system.